Satellite Evidence of Poles

Satellites that photograph high-latitude auroras have given us a visual record of the effective Earth magnetic dipole locations—they are not anywhere near the marks on most world maps. Charged particles, arriving from the Sun, follow paths toward the ionosphere defined by the Earth's distant field with its activity, seasonal, and day-night distortions. Most of this distortion of the Earth's dipole field disappears in the stronger field region inside 3 Earth radii. Fields from the multipole terms of the main field decrease faster with altitude than fields from the dipole terms. Thus, it is the eccentric axis dipole that is guiding the charged particles as they spiral closer to the Earth to excite a visible aurora. When solar-terrestrial disturbances are ripe for an entire polar auroral oval to be illuminated (Figure 3.14 and Plate 4), the average patterns (allowing for small day-night and solar-wind distortions) circle the eccentric axis dipole pole location, just northwest of Thule, Greenland, in the Northern Hemisphere and somewhat northeast of Vostok Station, Antarctica,

FIGURE 3.13 ► The polar region's locally measured vertical field position does not indicate the Earth's internal main field pole. A magnetometer positioned there also responds to the magnetic fields of high-latitude field-aligned currents, auroral region (ionospheric) currents, induced currents in the Earth, and special ocean-coast effects (in addition to the main field).

FIGURE 3.13 ► The polar region's locally measured vertical field position does not indicate the Earth's internal main field pole. A magnetometer positioned there also responds to the magnetic fields of high-latitude field-aligned currents, auroral region (ionospheric) currents, induced currents in the Earth, and special ocean-coast effects (in addition to the main field).

in the Southern Hemisphere. These are the pole locations with significant meaning.

The region where there are major satellite-memory upsets caused by bombarding particles (Figure 2.20) is identified with the low field values in the South Atlantic-South America region. This anomalous region (Figure 2.21) is a clear manifestation of the eccentric axis offset from the Earth's spin axis—

FIGURE 3.14 ► Every satellite picture of Earth's northern auroral oval appears to be centered near a pole location for the eccentric axis dipole. The midnight meridian is close to the right of this figure; sunlight can be seen as a crescent at the left. A coastline map of North America and Greenland is superposed on the image. Figure from NASA.

FIGURE 3.14 ► Every satellite picture of Earth's northern auroral oval appears to be centered near a pole location for the eccentric axis dipole. The midnight meridian is close to the right of this figure; sunlight can be seen as a crescent at the left. A coastline map of North America and Greenland is superposed on the image. Figure from NASA.

an important reason for recognizing the significance of the eccentric axis field representation.